[swift-evolution] Pre-proposal: Safer Decimal Calculations

[Haravikk]

> On 20 Mar 2016, at 17:54, Rainer Brockerhoff via swift-evolution <swift-evolution at swift.org> wrote:
> 
> On 3/20/16 14:43, Andrey Tarantsov via swift-evolution wrote:
>> I have no stake in this proposal, except for:
>> 
>>> I suggest, therefore, that this acceptance be indicated by an
>>> annotation to the literal; a form such as ~0.1 might be easiest to 
>>> read and implement, as the prefix ~ operator currently has no
>>> meaning for a floating-point value.
>> 
>> Whatever you do, don't touch the literals! I specify NSTimeIntervals
>> of 0.1, 0.2, 0.25 etc all over the place, and I couldn't care less if
>> my animations are one femtosecond off.
>> 
>> Don't pollute everyone's apps with tildes just because there's a
>> niche that needs to care about precision loss.
> 
> Yes, I'm now agreeing that the ideal solution is to leave the whole
> binary floating-point mess alone and write a reasonable `Decimal` type.
> I'll update my text accordingly when I find time.

The alternative would be to make the Decimal type the default anywhere that Double or Float isn’t used; while this may be overkill, it shouldn’t impact performance of most applications. At this point the less-safe but higher performance types would be opt-in by either specifying the type or using the tilde operator.

This could actually expand the definition of the ± on constants to instead imply “pick the type that can represent this”, so 0.1±0.01 would pick Double, Float or Decimal as appropriate, favouring the higher performance types only if they can represent that value without exceeding the allowable error.

It is a tricky area though, Swift does have a goal of safety so it may be worth pushing a change that promotes that, even if it means that means a few changes to get maximum performance back; this could be partly avoided by the way the change is handled though, providing warnings where the intent is ambiguous, or assuming performance over safety for existing code?